1. Field of the Invention
The present invention relates to a method of providing a location service using a mobile communication system.
2. Description of Related Art
A location system using a CDMA-based cellular phone system has been disclosed in a prior art reference, Japanese Patent Laid-Open Publication No. Hei 7-181242.
FIG. 25 shows a model of the above location system formed by three base stations located on a plane. An object 91 represents a mobile station and three base stations are labeled 921, 922, and 923, respectively. Coordinates (x, y) represent the position (coordinates) of the mobile station 91 and coordinates (x1, y1), (x2, y2), and (x3, y3) represent the positions (coordinates) of the base stations 921, 922, and 923, respectively. The base stations 921, 922, and 923 operate, based on common reference time T0. At the same time, the base stations 921, 922, and 923 have predetermined time offsets TO1, TO2, and TO3, respectively. The base stations 921, 922, and 923 repeats the transmission of same PN (pseudonoise) sequences at a same rate over their pilot channels. When the base stations 921, 922, and 923 transmit the PN sequences, the time at which the first pulse of the PN sequences is sent out (in other words, the time at which the transmission of the PN sequences starts) is delayed, according to the time offsets TO1, TO2, and TO3, respectively. Because the base stations also transmit their time offsets over their sync channels, respectively, the mobile station 91 can obtain the above time offsets TO1, TO2, and TO3. In addition, because the base stations transmit the time offsets of their neighboring base stations over their paging channels, the mobile station 91 can obtain the above time offsets TO1, TO2, and TO3. TP1, TP2, and TP3 respectively represent signal propagation time from the base stations 921, 922, and 923 to the mobile station. T1, T2, and T3 respectively represent the time when the first pulse of the PN sequences sent out from the base stations 921, 922, and 923 has arrived at the mobile station 91, measured at the mobile station 91
The position (coordinates) (x, y) of the mobile station 91 can be obtained by solving the following simultaneous equation where c is the velocity of light (x1, y1), (x2, Y2), and (x3, y3) are assumed to be known, but five values, x, y, TP1, TP2, and TP3 are unknown.(x−x1)^2+(y−y1)^2=(c×TP1)^2(x−x2)^2+(y−y2)^2=(c×TP2)^2(x−x3)^2+(y−y3)^2=(c×TP3)^2T2−T1=(TO2+TP2)−(TO1+TP1)T3−T1=(TO3+TP3)−(TO1+TP1)  (1)